All human beings live through a lifecycle – we are born, grow slowly to maturity, spend a long time as mature adults, then grow old, and die. During the early years, we consume goods and services that we do not produce. During the long middle years, we produce more than we consume, and in old age either we cease producing once more or else produce at a low level.
In human beings, it is the women who have the children. The men provide the sperm, talk trash, and puff their egos. There are obviously three possibilities. If on average each woman has two children (actually 2.01 children, but never mind), then the population will over the generations reproduce itself at a stable level. If each woman on average produces more than two children then the population will grow. If each woman on average produces fewer than two children then the population will shrink. These are not cultural facts or ideological musings or historical conjunctions, they are just biology and arithmetic.
A little while ago, I referenced a startling New York Times story which noted that because in China women produce on average roughly only 1.7 children, the enormous Chinese population of 1.4 billion souls is on track to shrink by the end of the 21st century to roughly 740 million. Notice by the way that 740 million is not some kind of point of stability. If the fertility rate in China remains 1.7, the population will continue to shrink as the decades go by. Indeed, bizarre though it may seem, other things being left to one side (more of this in a moment) a country with a 1.7 fertility rate will eventually cease to exist. Think about it. When the last 20 people – 10 women and 10 men – occupy what was once a country of 1.4 billion, if those 10 women have a fertility rate of 1.7 then in the next generation there will only be 17 Chinese. In the next generation that number will drop to 15, and so on until the last child buries his or her parents and wanders off into the sunset.
The population of the United States has been growing pretty steadily since I was born (and also, of course, before I was born.) But the fertility rate has been falling. In 1958, the fertility rate was 3.582. By 1972, it had fallen to 2.132, just barely replacement level. Currently, I was startled to discover when I went searching the web, the US fertility rate is 1.781, just slightly above that of China.
It is interesting to break out the US figures by race and ethnicity. White Americans have a fertility rate of 1.601, markedly below that of the Chinese. Black Americans are doing rather better with a fertility rate of 1.775, still well below replacement. The Hispanic rate is 1.94, close but no cigar. Asian Americans are at 1.51.
Children consume without producing, just as old people produce without consuming, but generally speaking in modern society children consume less per capita than old people so an aging shrinking population can put a severe strain on an economy. What to do?
Assuming that modern science will not figure out a way to get men pregnant, and in the face of what seems to be a broad international tendency for women to have fewer children, the only answer, at least in the short run, is for a country like the United States to get people to move here, especially young people of childbearing age.
In short, immigration is not the problem, it is the solution.